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One of the important sources for independent determination of stellar masses is eclipsing binaries with components on the main sequence, and with observable spectral lines of both components. The parameters of such stars are used to construct the mass–luminosity relation for stars of high and intermediate masses. Among them, the type of long-period eclipsing binaries stands out, the parameters of which are currently not fully determined, which is associated with the difficulties caused by the need for long-term observations. In this article, we will review the currently available observational data for such objects and discuss the prospects for their use to determine stellar masses.

Context. The JAXA Hayabusa2 mission will perform the first ever sample return from a primitive asteroid. The target near-Earth asteroid (162173) Ryugu will be reached in mid-2018 and its samples will be returned to the Earth by the end of 2020. Aims. We want to improve the current knowledge of the compositional and rotational properties of Ryugu, which are still presenting some uncertainties that might a ect the mission operations and scientific return. Methods. We acquired high-quality photometric time-series data with the FORS2 instrument at the Very Large Telescope of the European Southern Observatory (ESO-VLT, Chile). We also acquired four FORS2 visible spectra and three X-shooter spectra in the 0.35-2.15 μm range, at di erent rotational phases. Results. We obtained the currently highest-quality visual light-curve of Ryugu. A best solution of ∼7.63 h is found for the rotational period, while a short-period solution (i.e., P ≈ 3:8 h) is ruled out by the clearly non-symmetric light-curve. The obtained spectra are generally similar and featureless, but present a drop-o of the reflectance at <0.45 μm, suggesting the presence of aqueously altered minerals on Ryugu. The best meteorite analogs for Ryugu are represented by thermally altered CM carbonaceous chondrites. Conclusions. Our new photometric data help to refine the target reference model used by the Hayabusa2 team for the mission preparation and implementation, improving our knowledge of Ryugu's spin properties. Our new spectra constrain the compositional and geological context of the Ryugu's surface in order to prepare the planning of mission observations and support the working group for the selection of possible landing and sampling sites [ABSTRACT FROM AUTHOR]